Page No 143:

Question 1:

If a trait
A exists in 10% of a population of an asexually reproducing species
and a trait B exists in 60% of the same population, which trait is
likely to have arisen earlier?

Answer:

In
asexual reproduction, the reproducing cells
produce a copy of their DNA through some chemical reactions. However,
this copying of DNA is not accurate and therefore, the newly formed
DNA has some variations.

It
can be easily observed in the above figure that in asexual
reproduction, very few variations are allowed. Therefore, if a trait
is present in only 10% of the population, it is more likely that the
trait has arisen recently. Hence, it can be concluded that trait B
that exists in 60% of the same population has arisen earlier than
trait A.

Page No 143:

Question 2:

How does the creation of variations in a species promote survival?

Answer:

Sometimes for a species, the environmental conditions change so drastically that their survival becomes difficult. For example, if the temperature of water increases suddenly, most of the bacteria living in that water would die. Only few variants resistant to heat would be able to survive. If these variants were not there, then the entire species of bacteria would have been destroyed. Thus, these variants help in the survival of the species.

However, not all variations are useful. Therefore, these are not necessarily beneficial for the individual organisms.

Page No 147:

Question 1:

How do Mendel’s experiments show that traits may be dominant or
recessive?

Answer:

Mendel
selected true breeding tall (TT) and dwarf (tt) pea plants. Then, he
crossed these two plants. The seeds formed after fertilization were
grown and these plants that were formed represent the first filial or
F1
generation. All the F1
plants obtained were tall.

Cross-pollination of tall and short
plant

Then,
Mendel self-pollinated the F1
plants and observed that all plants obtained in the F2
generation were not tall. Instead, one-fourth of the F2
plants were short.

Self-pollination of F1
plants

From
this experiment, Mendel concluded that the F1
tall plants were not true breeding. They were carrying traits of both
short height and tall height. They appeared tall only because the
tall trait is dominant over the dwarf trait.

Page No 147:

Question 2:

How do Mendel’s experiments show that traits
are inheritedindependently?

Answer:

Since
the F1
plants are formed after crossing pea plants having green round seeds
and pea plants having yellow wrinkled seeds, F1
generation will have both these characters in them. However, as we
know that yellow seed colour and round seeds are dominant characters,
therefore, the F1
plants will have yellow round seeds.

Then
this F1
progeny was self-pollinated and the F2
progeny was found to have yellow round seeds, green round seeds,
yellow wrinkled seeds, and green wrinkled seeds in the ratio of
9:3:3:1.

Independent inheritance of two different traits

In the
above cross, more than two factors are involved, and these are
independently inherited.

Page No 147:

Question 3:

A man with blood group A marries a woman with blood group O and their daughter has blood group O. Is this information enough to tell you which of the traits − blood group A or O − is dominant? Why or why not?

Answer:

No. This information is not sufficient to determine which of the traits − blood group A or O − is dominant. This is because we do not know about the blood group of all the progeny.

Blood group A can be genotypically AA or AO. Hence, the information is incomplete to draw any such conclusion.

Page No 147:

Question 4:

How is the sex of the child determined in human beings?

Answer:

In human beings, the females have two X chromosomes and the males have one X and one Y chromosome. Therefore, the females are XX and the males are XY.

The gametes, as we know, receive half of the chromosomes. The male gametes have 22 autosomes and either X or Y sex chromosome.

Type of male gametes: 22+X OR 22+ Y.

However, since the females have XX sex chromosomes, their gametes can only have X sex chromosome.

Type of female gamete: 22+X

Sex determination in humans

Thus, the mother provides only X chromosomes. The sex of the baby is determined by the type of male gamete (X or Y) that fuses with the X chromosome of the female.

Page No 150:

Question 1:

What are the different ways in which individuals with a particular trait may increase in a population?

Answer:

Individuals with a particular trait may increase in a population as a result of the following:

(i) Natural selection: When that trait offers some survival advantage.

(ii) Genetic drift: When some genes governing that trait become common in a population.

Page No 150:

Question 2:

Why are traits acquired during the life-time of an individual not inherited?

Answer:

This happens because an acquired trait involves change in non-reproductive tissues (somatic cells) which cannot be passed on to germ cells or the progeny. Therefore, these traits cannot be inherited.

Page No 150:

Question 3:

Why are the small numbers of surviving tigers a cause of worry from the point of view of genetics?

Answer:

Small numbers of tigers means that fewer variations in terms of genes are available. This means that when these tigers reproduce, there are less chances of producing progeny with some useful variations. Hence, it is a cause of worry from the point of view of genetics.

Page No 151:

Question 1:

What factors could lead to the rise of a new species?

Answer:

Natural selection, genetic drift and acquisition of traits during the life time of an individual can give rise to new species.

Page No 151:

Question 2:

Will geographical isolation be a major factor in the speciation of a self-pollinating plant species? Why or why not?

Answer:

Geographical isolation can prevent the transfer of pollens among different plants. However, since the plants are self-pollinating, which means that the pollens are transferred from the anther of one flower to the stigma of the same flower or of another flower of the same plant, geographical isolation cannot prevent speciation in this case.

Page No 151:

Question 3:

Will geographical isolation be a major factor in the speciation of an organism that reproduces asexually? Why or why not?

Answer:

Geographical isolation prevents gene flow between populations of a species whereas asexual reproduction generally involves only one individual. In an asexually reproducing organism, variations can occur only when the copying of DNA is not accurate. Therefore, geographical isolation cannot prevent the formation of new species in an asexually reproducing organism.

Page No 156:

Question 1:

Give an example of characteristics being used to determine how close two species are in evolutionary terms.

Answer:

The presence of feathers in dinosaurs and birds indicates that they are evolutionarily related. Dinosaurs had feathers not for flying but instead these feathers provided insulation to these warm-blooded animals. However, the feathers in birds are used for flight. This proves that reptiles and birds are closely related and that the evolution of wings started in reptiles.

Page No 156:

Question 2:

Can the wing of a butterfly and the wing of a bat be considered homologous organs? Why or why not?

Answer:

The wing of a butterfly and the wing of a bat are similar in function. They help the butterfly and the bat in flying. Since they perform similar function, they are analogous organs and not homologous.

Page No 156:

Question 3:

What are fossils? What do they tell us about the process of
evolution?

Answer:

Fossils
are the remains of organisms that once existed on earth. They
represent the ancestors of plants and animals that are alive today.
They provide evidences of evolution by revealing the characteristics
of the past organism and the changes that have occurred in these
organisms to give rise to the present organisms.

Page No 158:

Question 1:

Why are
human beings who look so different from each other in terms of size,
colour and looks said to belong to the same species?

Answer:

A species is a group of organisms that are capable of interbreeding to produce a fertile offspring. Skin colour, looks, and size are all variety of features present in human beings. These features are genetic but also environmentally controlled. Various human races are formed based on these features. All human races have more than enough similarities to be classified as same species. Therefore, all human beings are a single species as humans of different colour, size, and looks are capable of reproduction and can produce a fertile offspring.

Page No 158:

Question 2:

In
evolutionary terms, can we say which among bacteria, spiders, fish
and chimpanzees have a ‘better’ body design? Why or why
not?

Answer:

Evolution
cannot always be equated with progress or better body designs.
Evolution simply creates more complex body designs. However, this
does not mean that the simple body designs are inefficient. In fact,
bacteria having a simple body design are still the most cosmopolitan
organisms found on earth. They can survive hot springs, deep sea, and
even freezing environment.

Therefore,
bacteria, spiders, fish, and chimpanzees are all different branches
of evolution.

Page No 159:

Question 1:

A Mendelian experiment consisted of breeding tall pea plants bearing violet flowers with short pea plants bearing white flowers. The progeny all bore violet flowers, but almost half of them were short. This suggests that the genetic make-up of the tall parent can be depicted as

(a) TTWW

(b) TTww

(c) TtWW

(d) TtWw

Answer:

(c) The genetic make-up of the tall parent can be depicted as TtWW

Since all the progeny bore violet flowers, it means that the tall plant having violet flowers has WW genotype for violet flower colour.

Since the progeny is both tall and short, the parent plant was not a pure tall plant. Its genotype must be Tt.

Therefore, the cross involved in the given question is

TtWw

×

ttww

↓

TtWw

−

ttww

Therefore, half the progeny is tall, but all of them have violet flowers.

Page No 159:

Question 2:

An example of homologous organs is

(a) our arm and a dog’s fore-leg.

(b) our teeth and an elephant’s tusks.

(c) potato and runners of grass.

(d) all of the above.

Answer:

(b)An example of homologous organs is all of the above.
Homologous organs are organs which are similar in structure and origin but have different functions in different organisms. Our arms and a dog's forelegs have same structure but perform different functions. Similarly our teeth and the tusks of elephants are similar in origin but perform different functions. Also potato and runners of grass are stem modifications. Hence, all the options are examples of homologous organs.

Page No 159:

Question 3:

In evolutionary terms, we have more in common with

(a) a Chinese school-boy.

(b) a chimpanzee.

(c) a spider.

(d) a bacterium.

Answer:

(a) In evolutionary terms, we have more in common with a Chinese
school boy.

Page No 159:

Question 4:

A study
found that children with light-coloured eyes are likely to have
parents with light-coloured eyes. On this basis, can we say anything
about whether the light eye colour trait is dominant or recessive?
Why or why not?

Answer:

Let us
assume that children with light-coloured eyes can either have LL or
Ll or ll genotype. If the children have LL genotype, then their
parents will also be of LL genotype.

LL

×

LL

↓

LL

If the
children with light-coloured eyes have ll genotype, then their
parents will also have ll genotype.

ll

×

ll

↓

ll

Therefore,
it cannot be concluded whether light eye colour is dominant or
recessive.

Page No 159:

Question 5:

How are the areas of study − evolution and classification − interlinked?

Answer:

Classification involves grouping of organism into a formal system based on similarities in internal and external structure or evolutionary history.

Two species are more closely related if they have more characteristics in common. And if two species are more closely related, then it means they have a more recent ancestor.

For example, in a family, a brother and sister are closely related and they have a recent common ancestor i.e., their parents. A brother and his cousin are also related but less than the sister and her brother. This is because the brother and his cousin have a common ancestor i.e., their grandparents in the second generation whereas the parents were from the first generation.

With subsequent generations, the variations make organisms more different than their ancestors.

This discussion clearly proves that we classify organisms according to their resemblance which is similar to creating an evolutionary tree.

Page No 159:

Question 6:

Explain the terms analogous and homologous organs with examples.

Answer:

Homologous organs are similar in origin (or are embryologically similar) but perform different functions. For example, the forelimbs of humans and the wings of birds look different externally but their skeletal structure is similar. It means that their origin is similar (as wings in birds are modifications of forearm) but functions are different - the wings help in flight whereas human forearm helps in various activities.

Homologous organs

Analogous organs, on the other hand, have different origin but perform similar functions. For example, the wings of a bird and a bat are similar in function but this similarity does not mean that these animals are more closely related. If we carefully look at these structures, then we will find that the wings of a bat are just the folds of skin that are stretched between its fingers whereas the wings of birds are present all along the arm. Therefore, these organs are analogous organs.

Page No 159:

Question 7:

Outline a project which aims to find the dominant coat colour in dogs.

Page No 159:

Question 8:

Explain the importance of fossils in deciding evolutionary relationships.

Answer:

Fossils are the remains of the organism that once existed on earth. They represent the ancestors of the plants and animals that are alive today. They provide evidences of evolution by revealing the characteristics of the past organisms and the changes that have occurred in these organisms to give rise to the present organisms. Let us explain the importance of fossils in deciding evolutionary history with the help of the following example.

Around 100 million years ago, some invertebrates died and were buried in the soil in that area. More sediment accumulated on top of it turning it into sedimentary rock.

At the same place, millions of years later, some dinosaurs died and their bodies were buried on top of the sedimentary rock. The mud containing dinosaurs also turned into a rock.

Then, millions of years later, some horse-like creatures died in that area and got fossilized in rocks above the dinosaur fossils.

Some time later, due to soil erosion or floods in that area, the rocks containing horse-like fossils are exposed.

If that area is excavated deeper, then the dinosaur and invertebrates fossils can also be found. Thus, by digging that area, scientists can easily predict that horse-like animals evolved later than the dinosaurs and the invertebrates.

Thus, the above example suggests that the fossils found closer to the surface of the earth are more recent ones than the fossils present in deeper layers.

Layers of fossils

Page No 159:

Question 9:

What evidence do we have for the origin of life from inanimate
matter?

Answer:

A
British scientist, J.B.S. Haldane, suggested that life originated
from simple inorganic molecules. He believed that when the earth was
formed, it was a hot gaseous mass containing elements such as
nitrogen, oxygen, carbon, hydrogen, etc. These elements combined to
form molecules like water (H2O),
carbon dioxide (CO2),
methane (CH4),
ammonia (NH3),
etc.

After
the formation of water, slowly the earth surface cooled and the
inorganic molecules interacted with one another in water to form
simple organic molecules such as sugars, fatty acids, amino acids,
etc. The energy for these reactions was provided by solar radiations,
lightning, volcanic eruptions, etc.

This was
proved by the experiment of Stanley L. Miller and Harold C. Urey in
1953.

They
took a mixture of water (H2O),
methane (CH4),
ammonia (NH3),
and hydrogen gas (H2)
in a chamber and sparks were passed through this mixture using two
electrodes. After one week, 15% of the carbon from methane was
converted into amino acids, sugars, etc. These organic molecules are
polymerized and assembled to form protein molecules that gave rise to
life on earth.

Miller and Urey experiment

Page No 159:

Question 10:

Explain how sexual reproduction gives rise to more viable variations than asexual reproduction. How does this affect the evolution of those organisms that reproduce sexually?

Answer:

In sexual reproduction, two individuals having different variations combine their DNA to give rise to a new individual. Therefore, sexual reproduction allows more variations, whereas in asexual reproduction, chance variations can only occur when the copying of DNA is not accurate.

Additionally, asexual reproduction allows very less variations because if there are more variations, then the resultant DNA will not be able to survive inside the inherited cellular apparatus.

However, in sexual reproduction, more variations are allowed and the resultant DNA is also able to survive, thus making the variations viable.

Variation and Evolution: Variants help the species to survive in all the conditions. Environmental conditions such as heat, light, pests, and food availability can change suddenly at only one place. At that time, only those variants resistant to these conditions would be able to survive. This will slowly lead to the evolution of a better adapted species. Thus, variation helps in the evolution of sexually reproducing organisms.

Page No 159:

Question 11:

How is the equal genetic contribution of male and female parents ensured in the progeny?

Answer:

In human beings, every somatic cell of the body contains 23 pairs of chromosomes. Out of these 23 pairs, the first 22 pairs are known as autosomes and the remaining one pair is known as sex chromosomes represented as X and Y.

Females have two X chromosomes and males have one X and one Y chromosome.

The gamete receives half of the chromosomes. Therefore, the male gametes have 22 autosomes and either X or Y chromosome.

The female gamete, on the other hand, has 22 autosomes and X chromosome.

During reproduction, the male and female gametes fuse and thus the progeny receives 22 autosomes and one X or Y chromosome from male parent and 22 autosomes and one X chromosome from the female parent.

Page No 159:

Question 12:

Only
variations that confer an advantage to an individual organism will
survive in a population. Do you agree with this statement? Why or why
not?

Answer:

In
species, variations that offer survival advantages
are naturally selected. Individuals adjust to their environments with
the help of these selected variations and consequently these
variations are passed on to their progeny. Evolution of organisms
occurs as a result of this natural selection.

However,
there can be some other variations, which do not offer any survival
advantage and arise only accidentally. Such variations in small
populations can change the frequency of some genes even if they are
not important for survival.

This
accidental change in the frequency of genes in small populations is
referred to as genetic drift.